In electricity power stations, a protective circuit breaker is to be found downstream from each generator. An isolating switch is provided in series with the circuit breaker and downstream therefrom in order to ensure that workers acting on the generator are fully protected. It is recalled that an isolating switch has the property of passing leakage currents to ground via its insulators and of not passing them between its contacts. Such a disposition is illustrated by the diagram of FIG. 1 which shows, in series, a circuit breaker D and an isolating switch S whose active portions are insulated from a grounded screened casing E by insulators IS. When the devices are open, leakage current if passes via the insulators and does not pass between the contacts of the isolating switch.
The Applicant has set out to provide a device where interruption takes place in a dielectric gas under pressure, which device is capable of performing the operations of a circuit breaker while simultaneously providing the protective functions offered by an isolating switch. This device is referred to as a "circuit breaker/isolating switch". It must have the interrupting power of a circuit breaker and it must also:
be compact and possess a minimum of parts so as to keep its cost price down; PA1 enable leakage currents to pass to ground via its insulators; PA1 enable the state of its contacts to be observed; PA1 include means for locking its contacts when the apparatus is disengaged; and PA1 provide adequate dielectric strength in the event of the pressure of its dielectric insulating gas falling suddenly.
Document FR-A-2 305 871 describes a circuit breaker/isolating switch including a dielectric screen which is connected to ground and which is situated between the contacts when they are in the open position.
Document FR-A-2 355 364 describes a circuit breaker including an observation porthole disposed in the casing of the device.